Field of the Invention
The present invention relates to a process for the recovery and regeneration of ammonia contained in the effluent obtained from a reaction zone where ammonia and oxygen are reacted with a paraffin to produce the corresponding aliphatic nitrile. In particular, the present invention relates to the recovery and regeneration of unreacted ammonia contained in the effluent passing from a reaction zone wherein ammonia and oxygen are reacted with (1) propane to produce acrylonitrile or (2) isobutane to produce methacrylonitrile.
U.S. Pat. Nos. 3,936,360 and 3,649,179 are each directed to a process for the manufacture of acrylonitrile utilizing propylene, oxygen and ammonia as the reactants. These gases are passed over a catalyst in a fluid bed reactor to produce acrylonitrile which passes from the reactor to a recovery and purification section. This reaction also has some unreacted ammonia which is typically removed from the process by treatment in the quench column with an acid. The '179 patent discloses that the quench acid may be either sulfuric, hydrochloric, phosphoric or nitric acid. The '360 patent teaches the use of sulfuric acid in the quench to remove the unreacted ammonia. In the manufacture of acrylonitrile using propylene as the hydrocarbon source, the preferred embodiments clearly utilize sulfuric acid with the resulting formation of ammonium sulfate. Typically,the ammonium sulfate is either recovered and sold as a co-product (fertilizer) or may be combined with other heavy organics produced in the process and deep-welled for environmentally safe disposal.
Great Britain Patent 222,587 is directed to ammonium recovery from an ammonia-containing gas mixture utilizing an aqueous phosphoric acid solution, an aqueous solution of ammonium hydrogen phosphate ((NH.sub.4)H.sub.2 PO.sub.4), or mixtures thereof. The ammonia is recovered by heat decomposition and dissolving the resulting residue in water to regenerate the ammonium recovery phosphate solution. This ammonia recovery process is directed to the recovery of ammonia from coal gas or coke ovens at temperatures of 50.degree. C. to 70.degree. C.
U.S. Pat. Nos. 2,797,148 and 3,718,731 are directed to the recovery of ammonia from a process stream used in the production of HCN. The process of recovery uses an ammonium phosphate solution to capture the ammonia and then uses steam stripping to regenerate the ammonia from the ammonium phosphate solution. Typically, the process is operated by contacting the ammonia-containing gas with a 25% to 35% by weight ammonium phosphate solution having a pH of about 6 at a temperature of between 55.degree. C. to 90.degree. C. Ammonia regeneration is affected by contacting the resulting ammonium phosphate solution with steam. The processes in each of these patents disclose that the ammonium ion/phosphate ion ratio is at least 1.2 or greater.
The process of the present invention is advantaged over the prior art practice used in propylene ammoxidation because it avoids the formation of an ammonium salt-containing waste stream that must either be (1) treated to recover the ammonium salt or (2) disposed of in an environmentally safe manner. Rather, the process of the present invention results in the recovery of ammonia and regeneration of the ammonium phosphate quench solution by subjecting the quench solution to elevated temperatures and pressure in order to decompose the ammonium phosphate salt. The quench system of the present invention results in additional unexpected advantages in propane ammoxidation compared to propylene ammoxidation to acrylonitrile. Among these advantages are: (1) complete capture of by-product acrolein, thus enhancing product recovery efficiency by minimizing loss of product through, for example, reaction of acrolein with HCN in the product separation and recovery train of the process, (2) lower TOC (Total Organic Carbon) in the quench bottoms, (3) higher percentage of organics present in the quench bottoms are present as strippable/recoverable monomers instead of unrecoverable waste polymers and (4) the ability to use a lower severity waste organic treatment (e.g., wet oxidation) because of the presence of lower TOC and polymers in the quench bottoms solution. A further significant advantageous feature of the process of the present invention is that all the waste water streams may be readily handled by conventional biotreatment processes unlike the waste streams associated with propylene ammoxidation to manufacture acrylonitrile.